Superconducting materials

ABSTRACT

A NbKAl type intermetallic compound superconducting material, in which part of Al is replaced with an element selected from a group consisting of Pb, Bi, Tl, Cu, Sb, Te, Y, Ce, Pr, Eu, Ca and Zn and, if necessary, part of Nb is replaced with Ta, is disclosed. This material has a critical temperature of 18*K or higher and a Beta -W type crystalline structure, with the value of K ranging between 2.8 and 4.0.

United States Patent n91 Kawabe et al.

[ June 3, 1975 SUPERCONDUCTING MATERIALS Inventors: Ushio Kawabe; Mitsuhiro Kudo;

Shigeo Fukase; Masato Ishibashi, all

of Tokyo, Japan Assignee: Hitachi, Ltd., Japan Filed: Dec. 1, 1972 Appl. Non 311,148

, Foreign Application Priority Data Dec. 1, 1971 Japan 46-96271 US. Cl. 75/174; 148/32; 148/32.5; 148/133; 335/216 Int. Cl. C22c 27/00; HOlf 1/04 Field of Search 335/216; 75/174; 148/32, 148/32.5, 133

References Cited UNITED STATES PATENTS 4/1966 Saur 75/174 X 12/1966 Rosi et al 75/174 X 12/1970 Sahm et a1. 75/174 OTHER PUBLICATIONS Physical Reviow Vol. 152, No. 1, Dec. 2, 1966 pgs. 341-344. Z. Metallkde, Bd59 (1968) N0. 9, Hunt & Ramm, pgs. 701-707.

Metallurgy of Advanced Electronic Materials, Vol. 19, (1962) Gordon and Breach Science Pub. N.Y. pgs. 71-86.

Primary ExaminerC. Lovell Attorney, Agent, or Firm-'-Craig & Antonelli [5 7] ABSTRACT 14 Claims, 9 Drawing Figures OBSERVED v DEBYE'S TEMPERATURE,6D K) PATENTEDJUH3 m 13,887,364

SHEET 1 016? FIG. I PRIOR ART 012- ,2=o.|o 3 ,r=o.|5 E 1 0.25

' FIG. 2

83L 36 Tm D- (Jo M 260 460 66o- CALCULATED DEBYES TEMPERATURE ,o (K) SHEET PATENTEDJUHQ 19 5 Y FIG. 3'

FIG. 4

SHEET PATENTED FIG.

X 8 w 1 HT 1 L |N. UO I EP L e\ MEPEEMQEMP FIG. 6

PRIOR ART Cu TYPE Pb TYPE MEDbEMmEMF Sb TYPE mmnknmwm 2m;

EUTECTOID TYPE SHEET PATENTED 3 FIG. 7

, 1 SUPERCONDUCTING MATERIALS BACKGROUND or THE INYIENTION 1. Field of the Invention This invention relates to useful'and novel superconducting materialsand. more particularly, to improved intermetallic compound superconducting materials having high critical temperature and a ,B-W type crystalline structure generally represented by a formula 1-1/ y)l\' la r)- BRIEF DESCRIPTION OF THE PRIOR ART characters are thought to be very suitable as the material for superconducting magnet coils and superconducting transmission cables. However, the critical temperature T of B-W type binary compounds is only as high as l8K in case of Nb Sn and NbaAl and no superconductivity at higher temperatures has heretofore been obtained in binary compounds.

SUMMARY OF THE INVENTION An object of the invention is to provide a novel superconducting material, which has high critical temperature and can be readily manufactured as practical material, and with which it is possible to alleviate the cooling conditions in its practical use.

To achieve the above-noted object of the invention, part of Al in the Nb Al type structure is replaced with another element M, and also part of Nb is, if necessary, replaced with Ta. The value of K is set within a range from about 2.8 to about 4.0. The element substituted for Al and the quantity of partial substitution may be about 3.2 atomic percent or less of Pb or Ce, about 3.8 atomic percent or less of Bi or Te, about 4.4 atomic percent or less of TI, about 6.4 atomic percent or less of Cu, about 5.8 atomic percent or less of Sb, about 4.2 atomic percent or less of Y, about 2.7 atomic percent or less of Pr or Eu, about 2.1 atomic percent or less of Ca, or about 7.2 atomic percent or less of Zn.

It is also effective, if necessary, to replace part of Nb with 4.0 atomic (at) percent or less of Ta. By substituting one of the aforementioned substitution elements for part of Al and if necessary Ta for part of Nb in the Nb Al type structure, it is possible to obtain compound superconducting materials having higher critical temperatures compared to the critical temperature (about l8K at the highest) of the usual B-W type binary superconducting compounds such as Nb Al. According to the invention, it is possible to obtain critical temperatures higher than I9.3K.

The invention is achieved as a result of detailed study and development of a McMillans theoretical treatment on stong-co'upled superconductors (W.Lz McMillan; Phys. Rev. 167 (1968), p. 331), and is based on the finding that it is possible to obtain an increased super conducting critical temperature T,. by converting a superconducting compound into a multi-element alloy in ,such a manner as to increase the averaged frequency BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing relations among critical temperature, averaged phonon frequency, pseudocoulomb potential and electron-phonon coupling constant for some superconducting materials.

FIG. 2 shows relations between calculated value and observed value of Debys temperature for some B-W type compounds.

FIG. 3 is a graph showing relations between electronphonon coupling constant and coefficient of electronic specific heat for some B-W type compounds.

FIG. 4 is a graph showing relations between coefficient of electronic specific heat of some B-W type compounds and' number of electrons per atom in the crystal.

FIG. 5 shows a model of C-D binary phase diagram with a showing of Debys temperature and coefficient of electronic specific heat for Nb c D, compounds plotted against x.

FIG. 6 shows four main types of binary diagram having an eutectic or eutectoid point.

FIG. 7 is a graph showing relations between critical temperature and concentration of element M for Nb (Al, M) alloys according to the invention.

FIG. 8 is a graph showing relations between critical temperature and concentration of Cu for (Nb, Ta) (Al, Cu) alloys according to the invention, with various concentrations of Ta.

FIG. 9 is a graph showing relations among critical temperature, K and x for Nb (Al, ,Zn alloys according to the invention.

DETAILED DESCRIPTION OF THE INVENTION The B-W type intermetallic compound Nb Al dealt with in the present invention is featured by a composite crystalline structure consisting of a body-centered cubic lattice formed by Al atoms, and Nb atoms lying in three perpendicular planes of the lattice. In this type of compound it is known that the critical temperature T is higher the closer the composition ratio thereof is to the stoichiometric one. Also, it is known that Nb-Nb atom chain in the B-W crystalline structure forms a very narrow d-band in the vicinity of the Fermi-level, thus increasing the Fermi-level d-electron state density and the superconducting critical temperature.

According to the McMillanss treatment (W.L. Mc- Millan; Phys. Rev. l67(l968)331) on strong-coupled superconductors characterized by strong coupling between phonon and electron, the critical temperature T. of a superconducting material is given as The averaged phonon frequency w is generally determined by the phonon spectrum, and in case of the B-W type structure it is approximately 1.3 times the Debys temperature The value of 6,, may be calculated from Lindemanns formula of melting point that holds well for simple metallic substances and alloys. In case of the B-W type crystalline structure, it is represented as where a., is the B-W type crystal lattice constant, T, is the melting temperature, and M, is the mean atomic weight.

FIG. 2 shows the correlation between observed Debys temperature and calculated Debys temperature for some known B-W type compounds. It will be seen that equation 2 holds fairly well although there is a tendency of forming clusters in dependence upon whether 3-d or 4-d band is occupied by the electrons of the element occupying the A site of intermetallic compounds represented by a general formula A C. Thus, it will be understood that to increase the value of 0 the compound should be converted into a multi-element material of a reduced lattice constant, an increased melting point and an increased molecular weight.

The value of p.* in equation 1) may be derived from the pseudo-coulomb potential theory, and in case of the B-W type structure it can be approximated by a constant of u*=0.13.

The third parameter A can be defined by the McMillans theory as mentioned earlier. It may also be obtained semiexperimentally from known data as shown in FIG. 3, in which the electron-phonon coupling constant and coefficient of electronic specific heat 'y are correlated for known B-W type compounds. As is shown, the relation between 'y/)\ and 'y is represented by a straight line, whose x or y-intercept differs in dependence upon the d-band that is occupied by the electrons of the element occupying the A site of the compound A C but whose slope is the same in either case. In this way, there holds a linear relation between 7 and A for a group of compounds having the same d-band structure. Thus, the value of A may be determined if the magnitude of y is determined. The parameter y may also be obtained from data as shown in FIG. 4, in which y of various B-W type compounds A C is plotted against the number Z of electrons per atom in the B-W type crystal, that is, the ratio between total electron number e and atom number a. In this case, the value of y greatly varies in dependence upon which of 3-d, 4-d and 5-d bands is occupied by the electrons of the high melting transition element occupying the A site. Also, it will be seen that the value of 7 continuously changes with Z depending upon the kind of element occupying the C site.

In accordance with the invention, in order to increase the critical temperature T either the A site or the C site of the B-W type binary superconducting compound A C is partially replaced with a suitable different element or both sites are replaced with suitable different elements to obtain multi-element superconducting compounds having T higher than that of the mother material.

The ,B-W type multi-element superconducting compounds obtained by incorporating two different foreign metals are generally represented as' The critical temperature of this type of materaial is greatly influenced not only by the kind of the substitution elements B and D but also by the composition ratios (.r, y and K) and various heat treatment conditions.

Taking Nb,,'(C, ,D,) as an example of the multielement compounds, to make T highest by selecting composition it is necessary to select K to be coincident with the stoichiometrical ratio, and best results may be obtained if it is in a range 2.8 K 4.0.

If the value of'K is less than 2.8, the superconducting character would be deteriorated due to generation of such compounds as Nb (C,D). The critical temperature T will be the highest with a value of K in the proximity of 3.0. With this value of K, however, the range of x with which it is possible to expect satisfactory results is very narrow. With greater values of K the critical temperature T may be increased for a broader range of X, but the highest value of T, would be reduced. If the value of K exceeds 4.0, the super conducting character will be deteriorated.

In another aspect, the concentration x of the element D should be selected to be in the proximity of the composition corresponding to the eutetic point of a binary eutectic alloy of C and D. The upper half of FIG. 5 is a model of C-D binary phase diagram, and the lower half of FIG. 5 shows the Debys temperature and coefficient of electronic specific heat for Nb C D, compounds plotted against x corresponding with the upper half. It will be seen from this FIGURE that the Debys temperature of the Nb (C,D) compound is the highest and the coefficient of electronic specific heat is large for the composition corresponding to the eutectic point of the elements occupying the C site. Accordingly, by selecting the concentration x of the element D in the system given as Nb -(C D to be in the proximity of a value for the composition corresponding to the eutectic point of the OD alloy, the superconducting character of Nb C compound may be improved particularly at the critical temperature thereof.

FIG. 6 shows four main types of phase diagrams of C and D, corresponding to A C and A -,D combinations, the D component occupying the C site in case of havng Nb as the element A and Al as the element C. Namely, there are Pb type, Cu type, Sb type with some varieties and eutectoid type. Of these combinations, those which can meet conditions for rendering into multi-element material having high T are Nb (Al-Pb), Nb (Al-Bi) and Nb (Al-Tl) as the Pb type, Nb (Al-Cu) as the Cu type, Nb (Al-Sb), Nb (Al-Te), Nb (Al-Y), Nb (Al- Ce), Nb (Al-Pr), Nb (Al-Eu) and Nb (Al-Ca) as the Sb type and Nb (Al-Zn) as the eutectoid type. In any of these materials the critical temperature T is higher compared to NbgAI.

For each of these combinations the aforementioned effective composition range applies. A concentration of the element D higher than the above range is undesired because reduction of T results.

It is to be noted that satisfactory results may be obtained not only with alloys where part of Al of the superconducting alloy represented as Nb Al is replaced with one of the aforementioned substitution elements.

takes a coordination number of 12 and whose atomic 5 radius is slightly smaller than that of Nb. In this case, the desired concentration x of the element D corresponding to the proximity of the eutectic point of the C-D alloy tends to be slightly influenced by the concentration y of the element Ta, but the resultant deviation may be empirically determined.

DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS The following examples are given in order for the in- 15 vention to be more fully understood.

EXAMPLE 1 This example deals with B-W type ternary superconducting materials represented by a formula Nb Al M where M is a member of a group consisting of Pb, Bi, Tl, Cu, Sb, Te, Y, Ce, Pr, Eu, Ca and Zn.

Samples of materials with compositions containing the above elements were obtained through a combination of plasma arc melting or sintering and levitation melting. Materials in the form of masses and with purities of at least'99.99 percent are weighed to prepare predetermined compositions, and the resultant compositions were subjected to plasma arc melting in an argon atmosphere under pressure of around 1 atm. to

produce button-like masses. These button-like masses were inverted several times for sufficient alloying. To obtain samples having a shape convenient for the measurement of T the button-like masses were subjected to levitation melting in an argon atmosphere, and the resultant fused materials were cast into water-cooled copper molds to producerod-like samples. In case of combinations including an element having sufficiently low melting point and vapor pressure compared to the high-melting transition element Nb, (for instance where M is Ca, Zn, Sb, Te, Ce, Pb or Tl), powdery materials were used, and predetermined compositions were sufficiently mixed and press molded with a pressure of l ton/cm. The resultant moldings were temporarily sintered in an argon atmosphere under a pressure of about 1.5 atm. and at a temperature of not higher than l,OO0C. to produce samples containing low-grade intermetallic compounds poor in Nb, which were then subjected to levitation melting as mentioned above to produce rod-like samples. 7

Many of the samples obtained in the above manner had a diameter of 3 mm and a length of 30 mm, but some samples were too brittle to have the length of 30 mm. These cast samples were wrapped with a Nb foil and sealed within a quartz ampoule evacuated to a vacuum degree of 10 Torr. for heat treatment at a predetermined temperature within a range between 600 and l,200C. for l to 15 days followed by sudden cooling with water and thus became aged materials. The stoichiometrical composition ratio K and atomic concentration x of the resultant samples were examined by chemical quantitative analysis, and the weight reduction of various components after the melting was within a range of several percent. Also, many of these samples were so brittle that they can be sufficiently crushed to study their crystal structure by the Debys X-ray refraction method. By this analysis it was found that the superconducting materials according to the invention with Al in the Nb Al partially replaced with the substitution element M showed typical refraction pattern of the B-W type crystalline structure for a range of .r up to nearly .r 0.25 in case where M is Ca, Te. Ce, Pr, Eu or T1 and for a range of .r up to nearly 0.5 in case where M is Cu. Zn. Sb. Y, Pb or Bi.

The critical temperature T.- of the samples obtained in the above manner was measured by the usual four resistance probe technique. More particularly, the critical temperature T is set to the temperature when the resistance recovered and just reaches one half of the difference of resistance between the superconducting state and normal state, wherein the sample is 30 mm long carries a constant current of several amperes per square centimeter between its ends.

FIG. 7 shows relations between critical temperature T and atomic concentration or atomic percent x of the element M for various Nb3Al M ternary alloys having been subjected to a heat treatment at a temperature of 700C. for hours. Where M is Pb, Ce, Bi, Te, Pr, Eu or Ca, the critical temperature T is the highest when about 1.25 atomic percent of Al is replaced with the third element M, that is, when x is set to about 0.05. The eutectic point of these Al-M systems lies between 1 and 5 atomic percent of the concentration of the element M, which substantially corresponds to the value of x of the element M when the critical temperature T is at its peak. Where M is Zn, Cu, Sb, Y or Tl, the critical temperature T is relatively high when up to about 5.0 atomic percent of Al is replaced with the element M, that is, for values ofx up to about 0.2, and the concentration x of the element M corresponding to the peak of the critical temperature T, substantially coincides with the corresponding concentration at the eutectic point of the Al-M system except for the case where M is Sb. The highest critical temperature obtained was 19.3K with Nb (Al,Zn).

From FIG. 7, Table 1 below lists the concentration range of the element M necessary to obtain T higher than about 18K, the concentration range of the element M necessary to obtain T higher than about 18.5l( and the most desirable concentration range of the element M to increase T The presence of about 0.05 atomic percent of M except for Sb leads to appreciable effects, while the presence of about 2 atomic percent of Sb leads to appreciable effects.

Table 1 Concentration range Concentration range Most desired Element of M (in atomic '71) of M (in atomic 71) concentration M for T, l8K for T l8.5K range of M (in atomic /1) Pb. Ce 31 0 2 2.8 0.9 1.6 Bi. Te 3.X 0 2 3.2 0.9 1.6 Ti 4.4 0 2 3.9 0.9 1.6 (u 64 06- 5.3 2.1-2.9

Table l Continued Concentration range Concentration range Most desired Element of M (in atomic '71) of M (in atomic 71) concentration M for T,. l8K for T l8.5K range of M (in atomic /1 Sh 5.8 4.6 5.4 Y 2.4. 3.2 4.2 0.6 1.9 0.9 L6 Pr. Eu 2.7 0.3 2.3 0.9 1.6 Ca 2.l 0.3 L9 0.9 1.6 Zn 7.2 0.2 5.8 0.9 1.6

EXAMPLE 2 This example deals with quarternary superconducting materials obtained by replacing part of Nb with Ta in the samples in the first example.

FIG. 8 shows relations between critical temperature T and concentration for Cu for B-W type intermetallic compounds according to the invention having composi- {ions of t).98 0.02)3 l-r r)a 0.95 0.0s)a i- ,Cu,) and Nb (Al .,Cu,). The samples in this embodiment were prepared as in Example 1.

it will be seen that the critical temperature of materials obtained by replacing part of Nb with Ta in the Nb (Al,M) system is higher, although slightly, compared to that of samples without incorporating Ta. The range of concentration of Ta to obtain the above effects is less than about 4.0 atomic percent.

EXAMPLE 3 This example deals with ternary superconducting materials obtained by changing the concentration of Nb in the samples in the first example.

FIG. 9 shows relations between critical temperature T. and concentration of Zn for alloys having B-W type crystalline structure and composition of Nb (Al .,Zn,) when K is 2.8, 3.0 and 4.0 respectively. The method of preparation of samples and method of measurement of the critical temperature are similar to the previous first and second examples.

This example reveals that alloys represented as Nb (Al .,M,) show high critical temperatures when K is within a range of 2.8 s K S 4.0. If the value of K is smaller than 2.8, no appreciable improvement of the critical temperature can be obtained. Also, where x representing the concentration of the element M substituted for Al is less than about 0.15, the critical temperature is highest with a value of K in the proximity of 3.0. For greater values of x, better results may be obtained with values of K greater than 3.0. However, with values of K greater than 4.0 the characteristics of the material becomes instable, and satisfactory results cannot be expected.

Similar results with respect to the value of K were obtained where M is other elements than Zn. The concentration of Nb corresponding to the above range of K is about 74 to 80 atomic percent. As has been mentioned in the second embodiment, less than about 4 percent of Nb may be replaced with Ta, and in such case the lower limit of the concentration of Nb is about 70 atomic percent.

As has been described above in detail, the superconducting materials according to the invention, said materials having B-W type crystalline structure and, multielement compounds which have novel composition of Nb-Al-M or Nb-Ta-Al-M (where M is a member selected from the group consisting of Pb, Bi, Tl, Cu, Sb, Te, Y, Ce, Pr, Eu, Ca and Zn) have higher critical temperature T, l9.3K at the highest) compared to that of the Nb Al materials and have characteristics sufficient for use as practical material.

What is claimed is:

l. A superconducting material having high critical temperature. said material being an alloy represented by the formula: Nb-Al-M, wherein M is an element selected from the group consisting of 0.053.8 atomic percent of Te, 0.05-2.4 atomic percent of Y. 3.2-4.2 atomic percent of Y. 0.05-3.2 atomic percent of Ce. 0.05-2.7 atomic percent of Pr, 0.05-2.7 atomic percent of Eu, 0.05-2.1 atomic percent of Ca and 0.05-7.2 atomic percent of Zn, wherein the amount of Nb is in a range of 74-80 atomic percent and the balance of said alloy is Al and very minor amounts of unavoidable impurities.

2. The superconducting material of claim 1, in which less than 4 percent of Nb is replaced with an equal amount of Ta.

3. The superconducting material of claim 1, in which M is 0.053.8 atomic percent of Te.

4. The superconducting material of claim 1, in which M is 0.05-2.4 atomic percent of Y.

5. The superconducting material of claim 1, in which M is 3.2-4.2 atomic percent of Y.

6. The superconducting material of claim 1, in which M is 0.05-3.2 atomic percent of Ce.

7. The superconducting material of claim 1, in which M is 0.05-2.7 atomic percent of Pr.

8. The superconducting material of claim 1, in which M is 0.05-2.7 atomic percent of Eu.

9. The superconducting material of claim 1, in which M is 0.052.l atomic percent sign of Ca.

10. The superconducting material of claim 1, in which M is 0.057.2 atomic percent of Zn.

11. A superconducting material having high critical temperature greater than 18.5K, said material being an alloy represented by the formula Nb-Al-M, where M is an element selected from the group consisting of 0.2-3.2 at percent of Te, 0.6-1.9 at percent of Y,

0.2-2.8 at percent of Ce, 0.3-2.3 at percent of Pr, 0.3-2.3 at percent of Eu, 0.3-l .9 at percent of Ca and 0.2-5.8 at percent of Zn, wherein the amount of Nb is in a range of 74-80 percent and the balance of said alloy is Al and very minor amounts of unavoidable impurities.

12. The superconducting material of claim 11, in which less than 4 percent of Nb is replaced with an equal amount of Ta.

13. The superconducting material of claim 11, in which said M is an element selected from the group consisting of 0.9-1 .6 at percent of Te, 0.9-1.6 at percent of Y, 0.9-1.6 at percent of Ce, 0.9-1 .6 at percent of Pr, 09-16 at percent of Eu, 0.946 at percent of Ca and 0.9-1.6 at percent of Zn.

3 ,887,364 9 l 14. A superconducting material having high critical 0.05 and 2.8 K 4.0 respectively, and the value temperature, said material being represented by a forof x approximately corresponds to the value of Z in an mula (N b,.,,Ta,,) (AI M wherein M is an element eutectic alloy represented by a formula Al,. M selected from the group consisting of Te, Y. Ce, Pr, Eu, Ca and Zn, and wherein the value of y and K are y 5 I 

1. A SUPERCONDUCTING MATERIAL HAVING CRITICAL TEMPERATURE, SAID MATERIAL BEING AN ALLOY REPRESENTED BY THE FORMULA: NB-AL-M, WHEREIN M IS AN ELEMENT SELECTED FROM THE GROUP CONSISTING OF 0.05-3.8 ATOMIC PERCENT OF TE, 0.05-2.4 ATOMIC PERCENT OF Y, 3.2-4.2 ATOMIC PERCENT OF Y, 0.05-3.2 ATOMIC PERCENT OF CE, 0.05-2.7 ATOMIC PERCENT OF PR, 0.05-2.7 ATOMIC PERCENT OF EU, 0.05-2.1 ATOMIC PERCENT OF CA AND 0.05-7.2 ATOMIC PERCENT OF ZN, WHEREIN THE AMOUNT OF NB IS IN A RANGE OF 74-80 ATOMIC PERCENT AND THE BALANCE OF SAID ALLOY IS AL AND VERY MINOR AMOUNTS OF UNAVOIDABLE IMPURITIES.
 1. A superconducting material having high critical temperature, said material being an alloy represented by the formula: Nb-Al-M, wherein M is an element selected from the group consisting of 0.05-3.8 atomic percent of Te, 0.05-2.4 atomic percent of Y, 3.2-4.2 atomic percent of Y, 0.05-3.2 atomic percent of Ce, 0.05-2.7 atomic percent of Pr, 0.05-2.7 atomic percent of Eu, 0.05-2.1 atomic percent of Ca and 0.05-7.2 atomic percent of Zn, wherein the amount of Nb is in a range of 74-80 atomic percent and the balance of said alloy is Al and very minor amounts of unavoidable impurities.
 2. The superconducting material of claim 1, in which less than 4 percent of Nb is replaced with an equal amount of Ta.
 3. The superconducting material of claim 1, in which M is 0.05-3.8 atomic percent of Te.
 4. The superconducting material of claim 1, in which M is 0.05-2.4 atomic percent of Y.
 5. The superconducting material of claim 1, in which M is 3.2-4.2 atomic percent of Y.
 6. The superconducting material of claim 1, in which M is 0.05-3.2 atomic percent of Ce.
 7. The superconducting material of claim 1, in which M is 0.05-2.7 atomic percent of Pr.
 8. The superconducting material of claim 1, in which M is 0.05-2.7 atomic percent of Eu.
 9. The superconducting material of claim 1, in which M is 0.05-2.1 atomic percent sign of Ca.
 10. The superconducting material of claim 1, in which M is 0.05-7.2 atomic percent of Zn.
 11. A superconducting material having high critical temperature greater than 18.5*K, said material being an alloy represented by the formula Nb-Al-M, where M is an eleMent selected from the group consisting of 0.2-3.2 at percent of Te, 0.6-1.9 at percent of Y, 0.2-2.8 at percent of Ce, 0.3-2.3 at percent of Pr, 0.3-2.3 at percent of Eu, 0.3-1.9 at percent of Ca and 0.2-5.8 at percent of Zn, wherein the amount of Nb is in a range of 74-80 percent and the balance of said alloy is Al and very minor amounts of unavoidable impurities.
 12. The superconducting material of claim 11, in which less than 4 percent of Nb is replaced with an equal amount of Ta.
 13. The superconducting material of claim 11, in which said M is an element selected from the group consisting of 0.9-1.6 at percent of Te, 0.9-1.6 at percent of Y, 0.9-1.6 at percent of Ce, 0.9-1.6 at percent of Pr, 0.9-1.6 at percent of Eu, 0.9-1.6 at percent of Ca and 0.9-1.6 at percent of Zn. 